8 Reasons Why Grey Hair Happens - Even On The Young!

In rare cases, gray hair could be caused by an autoimmune disease.
An autoimmune disease like alopecia areata causes your immune system to attack your hair follicles and prevent them from growing, and "sometimes, in rare conditions, when the hair grows back, it grows back white," Dr. Day explains.Vitamin B-12 deficiencies caused by autoimmune diseases like autoimmune thyroiditis (aka Hashimoto's disease) have also been linked to premature graying. But Dr. Day notes that there isn't enough research to prove a clear cause and effect.